Contact angle, i.e., the angle between a solid surface and the liquid-air interface for a liquid droplet on the solid surface, measured through the liquid phase, is a fundamental quantity in surface science1,2. Contact angle is of practical importance for characterising the wettability of solid surfaces to provide information about surface heterogeneity and roughness, solid surface energy, and liquid spreading2-8. As such, knowing contact angle value is important for technological advancement, or scientific understanding, in a variety of areas such as: the chemical industry, the development of coatings, understanding froth floatation, the pharmaceutical industry, petroleum recovery, polymer testing, the printing industry, the semiconductor industry, the paper industry, and development of adhesives8-12. The unit for specifying contact angle is degrees.
Surface (or interfacial) tension is a manifestation of the imbalance of molecular forces that any interface between two bulk phases (e.g., solid-liquid or liquid-gas) experiences. Knowing the value of surface tension provides many insights for various topics, e.g., adsorption rate of surfactants, spreading coefficient, stability of an interface, possibility of coalesce, emulsions formulation, just to name but a few. As such, knowing the value of the surface tension is important for many scientific/industrial fields, e.g., detergency, petroleum refining, the polymer industry, biomedical engineering, the paint industry, ink formulations, cosmetics, the food industry, textiles, etc.1,13. The unit for specifying surface tension unit is J/m2 or N/m.
In view of the above, having an instrument to determine the value of contact angle and surface tension has a very broad appeal and application to academia and a wide range of industries, including related research and development or quality assurance. It is also clear that training students to be able to measure and interpret such data can be important for training of chemists, chemical and material engineers, as well as practitioners of other science and engineering disciplines. The knowledge of sliding or rolling angle is also very important for study of drop shedding and drop/bubble adhesion17. Analysis of constrained sessile drops has also shown great potential for study of systems with surfactants where very low surface or interfacial tension is expected18. However, commercially available instruments capable of determining the value of contact angle and surface tension often cost several thousands of dollars14. As such, this equipment is not sufficiently inexpensive to be used for training of students in the numbers that are needed in a teaching lab. Having a measurement instrument available at a lower price could open or create additional educational and training markets.
Contact angle measurement techniques are mainly divided into two categories. The first category, force tensiometry methods, measures the liquid-solid interaction force, and relates it to the contact angle through the Young equation2, e.g., using the Wilhelmy plate method. The second category is optical tensiometry methods, where contact angle is measured directly, e.g., using a droplet placed on a solid surface15, and deciphering the contact angle from the droplet/bubble image through Drop Shape Analysis (DSA) (e.g., by image processing methods to find the drop/bubble outline and then determining the contact angle by fitting the drop outline to a circle, ellipse, polynomial, or through a solution of the Laplace equation, or an augmented Laplace equation, e.g., in circumstances when an electrical field is present). Since the early 1980s, digital image processing methods have been used for DSA16, and such methods have continuously improved. A drawback of the force tensiometry methods is that they rely on the applicability of the Young equation. However, in most practical cases an apparent (macroscopic) contact angle is seen or used, which is different from the equilibrium contact angle needed for the Young equation; hence, optical methods are preferred19. Other advantages of optical methods include a small sample liquid requirement and the applicability of optical methods to surface samples of different sizes or shapes.
Similar to contact angle measurement, surface tension can be measured with force tensiometry methods, e.g., the duNouy ring method. However, DSA methods are popular due to their advantages of being a non-contact method, requiring small sample sizes, accuracy, convenience, versatility (e.g., the ability to conduct static or dynamic measurements), and ease of reconfiguration for high pressure or temperature measurements. In DSA methods, an image is taken from a drop or bubble—usually a pendent drop, captive bubble, or a constraint sessile drop, or a drop on an inclined surface—and the drop or bubble's outline is found by image processing, and then it is fitted to the numerical solution of the Laplace equation. The best theoretical representation of the experimentally found (imaged) profile will allow surface/interfacial tension to be found. The theoretical solutions of the Laplace equation are found by assuming a surface or interfacial tension value, and knowledge of a number of other parameters, such as the densities of fluids involved, as described for example in Applied Surface Thermodynamics, 2nd Edition, A. W. Neumann, R David, Y Zuo, 2010, CRC Press, NY, USA.
Since their advent in the 1980s, computerized DSA systems have generally had a standard system design as illustrated in FIG. 1. A droplet 102 is positioned by a syringe 112 (either from bottom or top) between a camera 104 connected to a lens 106 and a light source 108. An optical diffuser 110 may be positioned between the light source 108 and the droplet 102. The camera 104 is then connected to an external computer and monitor (not shown). This arrangement lends itself to a bulky system as bound to be placed on a table. Industry demand for on-site (field) measurements has very recently compelled a few manufacturers to offer smaller, transportable versions of such instruments at prices exceeding $15,000 USD and which are still tethered to a computer20 and cannot be used to measure surface tension of liquids.